Fibroblasts and endothelial cells both are capable of the phagocytosis and degradation of mast cell granules as demonstrated by microscopy and by the use of radiolabeled mast cell granules. Mast cell granules rapidly and selectively degrade extracellular fibronectin. While this degradation is due to chymase, mast cell granules are particularly efficient at cleaving fibronectin by virtue of their heparin content. This may represent a major extracellular function of mast cell granules and influence repair mechanisms within connective tissues. Proteoglycan heparin is degraded within minutes of exposure to reactive radicals formed during the respiratory burst. The products have an approximate molecular weight of 12,000, which is similar to the size of heparins in commercial preparations. The cleavage product retains anticoagulent activity. HL60 cells and their eosinophil- and neutrophil-like progeny all produce chondroitin 4-sulfates, but substantially differ in the rate at which they synthesize and degrade these molecules. Human mast cells and elevated histamine levels can be found in the synovial fluid of patients having a wide variety of arthritides including rheumatoid arthritis, systemic lupus erythematosis, and osteoarthritis. These mast cells contain tryptase and appear to be connective tissue in type.